Estrogen is the hormone that causes the bitch to act receptive. Estrogen causes the vulvar swelling, the bleeding, and the changes in the vagina that many veterinarians use to do "smears." In the average bitch, estrogen rises anywhere from 4 to 20 days before ovulation. Because of this very wide range, estrogen levels are only a very rough estimate of when the bitch will be fertile. Estrogen plays no actual role in causing ovulation.
LH is luteinizing hormone. LH actually causes ovulation. It is normally at very low levels in the blood, then suddenly surges and drops back to near zero again. Two days later, ovulation occurs, and then the eggs must mature for two more days before they become capable of being fertilized. The eggs are mature for two to three days, after which they deteriorate and die if they have not been fertilized. Therefore, if you considered the LH peak to be day zero, the eggs can be fertilized four to seven days later.
It is possible to test for the LH level, and this is actually the most accurate way to do it. However, since LH is only elevated for a short time (as little as 18 hours), daily blood testing is required, so this isn’t nearly as practical as our other alternative, progesterone testing.
Progesterone is also normally at very low levels, then as the heat progresses, progesterone will rise fairly abruptly. It remains elevated for 8-9 weeks (the duration of a pregnancy) whether or not the bitch is pregnant or was bred. At the end of the pregnancy (or non-pregnancy), the progesterone drops abruptly. This sudden drop causes labor to start and is also responsible for the symptoms of false pregnancy, which are often seen in dogs. In fact, all bitches to go through false pregnancy with every heat cycle, some just exhibit signs more blatantly than others do.
The way we use progesterone for ovulation timing is to look at the initial rise from baseline levels. This rise occurs within a day or two of the LH surge. There is some variation, but the general rule is that the rise in progesterone from below 2.0 to above 2.0 occurs approximately at the time of the LH surge. The level will reach about 5.0 the day of ovulation and should be in the 8.0 to 12.0 range when the eggs are fully mature. Because progesterone continues to rise (verses a quick surge) it can be done every two to three days in most cases. This type of ovulation timing is used for many reasons…fresh chilled and frozen breeding, infertility evaluations, natural breeding with limited stud access, and breedings with older males or dogs with poor semen quality.
Typically we like to pull progesterone levels on Day 5 of the bitches heat cycle. Then we will pull blood every other day until we see the LH surge, and then we will determine when we need to breed.
When pulling a progesterone sample, it is very important that the blood NOT be hemolyzed. Hemolysis is breakage of erythrocytes is easy to detect, as the hemoglobin turns the serum or plasma sample from pink to red, depending on the number of cells that have lysed. Hemolysis is a primary driver of the need for re-draws. It caused the progesterone results to be altered, which means we do not breed at the correct time. This is why we HIGHLY recommend that we pull all progesterone samples at our office.